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Using an AS-Path Access List

The following commands assign weights to routes filtered by AS-path access lists on router LA:



host1(config)#router bgp 400 

host1(config-router)#neighbor 10.5.5.1 remote-as
100 

host1(config-router)#neighbor 10.5.5.1 filter-list
1 weight 1000 

host1(config-router)#neighbor 10.72.4.2 remote-as
300 

host1(config-router)#neighbor 10.72.4.2 filter-list
2 weight 500 

host1(config-router)#exit 

host1(config)#ip as-path access-list 1 permit
^100_ 

host1(config)#ip as-path access-list 2 permit
^300_

Access list 1 permits any route whose AS-path attribute begins with 100 (specified by ^). This permits routes that pass through router Boston, whether they originate in AS 100 (AS path = 100) or AS 200 (AS path = 100 200) or AS 300 (AS path = 100 200 300). Access list 2 permits any route whose AS-path attribute begins with 300. This permits routes that pass through router NY, whether they originate in AS 300 (AS path = 300) or AS 200 (AS path = 300 200) or AS 100 (AS path = 300 200 100).

The neighbor filter-list commands assign a weight attribute of 1000 to routes passing through router Boston and a weight attribute of 500 to routes passing through router NY. Regardless of the origin of the route, routes learned through router Boston are preferred.

ip as-path access-list

Use to define a BGP access list; use the neighbor filter-list command to apply a specific access list.
You can apply access list filters on inbound or outbound BGP routes, or both.
You can permit or deny access for a route matching the condition(s) specified by the regular expression.
If the regular expression matches the representation of the AS path of the route as an ASCII string, then the permit or deny condition applies.
The AS path allows substring matching. For example, the regular expression 20 matches AS path = 20 and AS path = 100 200 300, because 20 is a substring of each path. To disable substring matching and constrain matching to only the specified attribute string, place the underscore (_) metacharacter on both sides of the string, for example _20_.
The AS path does not contain the local AS number.
Use the no version to remove a single access list entry if permit or deny and a path-expression are specified. Otherwise, the entire access list is removed.
See ip as-path access-list.

neighbor filter-list

Use to apply an AS-path access list to advertisements inbound from or outbound to the specified neighbor, or to assign a weight to incoming routes that match the AS-path access list.
You can specify an optional weight value with the weight keyword to assign a relative importance to incoming routes matching the AS-path access list.
The name of the access list is a string of up to 32 characters.
You can apply the filter to incoming or outgoing advertisements with the in or out keywords.
If you specify a BGP peer group by using the peerGroupName argument, all the members of the peer group inherit the characteristic configured with this command unless it is overridden for a specific peer. However, you cannot configure a member of a peer group to override the inherited peer group characteristic for outbound policy.
New policy values are applied to all routes that are sent (outbound policy) or received (inbound policy) after you issue the command.
To apply the new policy to routes that are already present in the BGP routing table, you must use the clear ip bgp command to perform a soft clear or hard clear of the current BGP session.

Behavior is different for outbound policies configured for peer groups for which you have enabled Adj-RIBs-Out. If you change the outbound policy for such a peer group and want to fill the Adj-RIBs-Out table for that peer group with the results of the new policy, you must use the clear ip bgp peer-group command to perform a hard clear or outbound soft clear of the peer group. You cannot merely perform a hard clear or outbound soft clear for individual peer group members because that causes BGP to resend only the contents of the Adj-RIBs-Out table.
Use the no version to remove the filter list.
See neighbor filter-list.

neighbor weight

Use to assign a weight to a neighbor connection.
All routes learned from this neighbor will have the assigned weight initially.
The route with the highest weight will be chosen as the preferred route when multiple routes are available to a particular network.
The weights assigned with the set weight commands in a route map override the weights assigned with the neighbor weight and neighbor filter-list commands.
If you specify a BGP peer group by using the peerGroupName argument, all the members of the peer group inherit the characteristic configured with this command unless it is overridden for a specific peer.
New policy values are applied to all routes that are sent (outbound policy) or received (inbound policy) after you issue the command.
To apply the new policy to routes that are already present in the BGP routing table, you must use the clear ip bgp command to perform a soft clear or hard clear of the current BGP session.

Behavior is different for outbound policies configured for peer groups for which you have enabled Adj-RIBs-Out. If you change the outbound policy for such a peer group and want to fill the Adj-RIBs-Out table for that peer group with the results of the new policy, you must use the clear ip bgp peer-group command to perform a hard clear or outbound soft clear of the peer group. You cannot merely perform a hard clear or outbound soft clear for individual peer group members because that causes BGP to resend only the contents of the Adj-RIBs-Out table.
Use the no version to remove the weight assignment.
See neighbor weight.

See Access Lists for more information about using access lists.

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